Means for controlling a nuclear reactor



Dec. 17, 1957 v. c. wlLsoN ETAL I 2,816,860`

MEANS FOR CONTROLLING A NUCLEAR RECTOR Filed Ap'rl 14, 1945 8 Sheets-Sheet l 6a??? ze @s Dec. 17, 1957 v. c. wlLsoN l- TAL 2,816,360

I MEANS FOR CONTROLLING A NUCLEAR REACTOR Filed April 14, 1945 8 Sheets-Sheet 2 Dec. 17, 1951 v. c. wxLsoN ErAL MEANS FOR CONTROLLING A NUCLEAR. REACTOR 8 Sheets-Sheet 3 Filed April 14, 1945 Dec. 17, 1957 v v. c, wlLsoN ETAL MEANS FOR coNTRoLLrNG A NUCLEAR REAcToR sheets-sheet 4 Filed April 14, 1945 w1 AAN 8 Sheets-Sheet 5 Dec. 17, 1957 v v. c. wlLsoN ErAL n MEANS FOR CONTROLLING A NUCLEAR REAcToR Filed April 14, 1945 8 Sheets-Sheet 6 Dec. 17, 1957 v. c. WILSON ETAL MEANS FOR CONTROLLING A NUCLEAR REACTOR Filed April 14, 1945 Dec. 17, 1957. v. c. wlLsoN ErAL 2,816,860

MEANS FOR CONTROLLING A NUCLEAR REACTOR Filed April 14, 1945 8 Sheets-Sheet '7 Dec. 17, 1957 v. c. WILSON rAL MEANS FOR CONTROLLING A NUCLEAR REAcToR Filed April 14, 1945 8 Sheets-Sheet 8 @Emi MEANS nonr coN'rRoLLING A NUCLnAR RnAcroR Volney C. Wilson, Santa Fe, N Mex., Wilcox P. Overbeek, Richland, Wash., LonisSlotimfSanta Fe, N. Mex., and Darol K. ,Fromang- Denver, .'Colo assgnors. to the United States ofAmerica as represented by the United States Atomic Energy. Commission Application-April 14,1945, Serial No. 588,302

.11 Claims. (Cl. 204--193.2)

forcontrol of self-sustainingnuclear ssion chain react- .ing systems.

Certain isotopes, such as U233 and-U235 in natural uranium, 94239 orI other. isotopes of element 94, .can be. split or iissioned by bombardment with thermal neutrons, .i. e.

neutrons in thermal .equilibrium with the `surrounding medium. Due to this phenomenon, a `self-sustaining chain reacting system operating at high neutronldensities can be built. In such Aa system, the ssion neutrons .pro-

yduced give rise to new fission neutrons in suciently -large numbersv tov overcome the neutron losses in-the system. Since the result of the fission of the uranium or similar nucleus is the production of lighter elements with great kinetic energy, plus approximately 2 neutronsfor.. each fission, along with beta and gamma radiation, la.- large amount of power in the form of heat .can be made available. A self-sustaining chain reacting system has been described in U. S. Patent 2,708,656 issued to Fermi etal. on May 17, 1955.

Most of the neutrons arising from the fission yprocess are lset free with a very high energy of .above onemillion electron volts average and are therefore not in condition to be utilized most etliciently to createnew yissions in the U235, when it is mixed with a .considerable quantity of D238, particularly as in the case of natural uranium. The 'energies of the fission-released fast neutrons are so highthat most of the latter would tend to be absorbed by the U23? nuclei, and yetfthe energies are not generally high enough for production of ssionby more .than a small fraction of the fast neutrons absorbed. :For neutrons of thermal energies, however, the absorption crosssection of U235, to produce fission, rises a greatdeal more than the simple capture cross-section of Um; .so that under the stated circumstances vthe fast fission neutrons, after they are created, are slowed down to thermalen- 'ergy, the energy'at which they aremost effective to produce fresh fission by bombardment ofadditional U235 atoms. ylf a system is made in which neutro-ns are slowed down without much absorption until theyreachthermal energies and then mostly enter into uranium rather vthan into any other element, a self-sustaining.nuclear chain 'reactionis obtained, even Vwith natural-uranium. Light "'ele'ments,"such as deuteriurn in the form ofheavy water,

beryllium, oxygen, ory carbon, the latter in the form of graphite, can be used as slowing .agents or moderators. A'specal advantage of the use ofthelight elements "even ifor thermal neutrons. Carbon-A intheform- -ot graphite is airelatively inexpensive, practical,. and yreadily mentioned for slowing down fast fission neutrons isthat .the order-mentioned.

2,3 l bb@ Patented Dec. l 7, l. 957

2` available vagent yfor slowing fastneutrons to thermal energies." Recently beryllium has .become available in ySulliciently'large quantities for test as to suitabrhty foruse as `a neutron vslowing material in a system oithe type to be described-- It has been found to be in. every waysatis- `factory.

However, in order for theaprornise to benfullledfthat the fastfission neutrons be. slowed toV thermalenergy a.-slowing medium without too large tan absorption 1n A'the uranium, every possible precaution is taken..to..con

.serve-neutronsffor the .chain reaction.

The ratioof the number of fast neutrons produced. in each generation bythe issions to the original number of fast neutrons creating -the ssions in a system of infinite gsize, -using specific materials, is called themultiplication .constant Vot 'the system and isdenoted by vthe symbol K.

If K L can be rnade sufficiently greater than unity toereate la net gai-n in neutrons, andthe system madesutliciently large so that this gain is-not entirely'lost by leakage from `thefexterior surface of the system,-then a self-sustaining;. ehain @reacting` system will vproduce power by .nuclear fission of natural uranium. The neutron reproduction ratio r in a system of finite size-differs from K bylthe` exterior .leakage factor and. by `a factor represent- `ing a .l oss ydue to localized absorbers lWithin the .reactor at` any one time The l.reproductionratiojmust be .Sufficiently greater ythan unity to permit -theneutrondensity torrise exponentially within the syst em.;--;Such rise..will

4tend to continue indefinitely if-notfcontrolfled at a desired `density.wrresno.Udine to;adesiredfpowerfoutput- It is :therefore :important to construct asystem-com- Aprising uranium and .a slowing. m edium so thatmeutron losses are reduced to such an extent that a controllable .self-sus t aining neutron chain-iiss ionreaction isobtained therein, with resultant regulated lproduction of;.neutrons, ,liberation of I.power .in the ,form of 'heatA and` ,production of radioactive ssion products and ,newl elements bothradioactive .and stableproduced by thel absorption o f neutrons.

It isan objectfof our. invention to provide l: r ct ntrol system responsive to the `neutron :density in ayportion of a self-sustaining neutron chainreactingsystem.for automatically maintaining. the .total poweroutput aL- a subst antially constant predetermined value.

Since changes in neutron density .can,funder -cert ain conditions, occur ,with Igreat.. rapid ity, lit isiintended to provide a control system ,responsivenotonly to.. th e pre vailing neutron density, butalso `responsive tofthevfrate of change of neutron density. Morze.partieularly, the time lag inherent yin theoperaton ofthecontrol mechanism would ordinarilybe s uch vthat vthe .neutron A density 'might suddenly lrise to .abnormally high svllllesiffbefore control could beelected. It is thereforeanlobjeet-of our invention to vprovide rateresponsivefmeans At`o1j--causing the control mechanism to. overrun with thefonset of a relatively suddenvchange so as to. cause a--.co rnpens at ing control function v.before the changehasfully,takenyplace Another object ofour invention is to-.change the reproduction ratio of a chainreacting system.tOfypeimit` an increase and decrease 'of power output.

f Afnrther'object of ourinvention is to p1f0vid e sa tety means, 1n the `aforesaldcontrol systemresponsive Vt o .,r1eu

'tron-density, for keeping thepower .foutput,below apreldetermm'ed maximum value atA all times.

. During-'the interchange of neutrons in a systemlcomout-. of the.-is.ystem. These losses-fwill--be considered in astaseo Natural uranium, particularly by reason of its U238 content, has an especially strong absorbing power for neutrons when they have been slowed down to so called resonance energies. The absorption in uranium at these energies is termed the uranium resonance absorption or capture. It is caused by the isotope U238 and does not result in fission but leads to the creation of the relatively stable nucleus 94239. It is not to be confused with absorption or capture of neutrons by impurities, referred to later. Neutron resonance absorption in uranium may take place either on the surface of the uranium bodies, in which case the absorption is known as surface resonance absorption, or it may take place in the interior of the uranium body, in which case the absorption is known as volume resonance absorption. It will be appreciated that this classification of resonance absorptions is merely a convenient characterization of observed phenomena, and arises, not because the neutron absorbing power of a U2?8 nucleus is any greater when the nucleus is at the surface of a body of metallic, or combined uranium, but

vbecause the absorbing power of U238 nuclei for neutrons `collisions inside the uranium body and may thus arrive at resonance energies therein, to be absorbed directly and immediately by Um. After successfully reaching thermal velocities, neutrons are also subject to capture by U238 without fission. Such capture by U238 produces 92239 leading7 to the production of 94239 by beta decay.

Thermal neutrons are also subject to capture by the moderator. While carbon and beryllium have very small capture cross-sections for thermal neutrons, and deuterium extremely small, a fraction of thermal neutrons is lost by capture in the slowing material during diffusion therethrough. It is therefore desirable to have the neutrons reaching thermal energy promptly enter uranium.

In addition to the above mentioned losses, which are inherently a part of the nuclear chain reaction process, impurities present in both the slowing material and the uranium add a very important parasitic neutron loss factor in the chain. The effectiveness of various elements as neutron absorbers varies tremendously. Certain elements, such as boron, cadmium, samarium, gadolinium, and some others, if present even in a few parts per million, could very likely prevent a self-sustaining chain reaction from taking place. It is highly important, therefore, to remove as far as possible all impurities capturing neutrons to the detriment of the chain reaction from both the moderator and the uranium. If these impurities, solid, liquid or gaseous, and in elemental or cornbined form, are present in too great quantity in the uranium bodies or the slowing material or in, or by absorption from the free spaces of the system, the self-sustaining chain reaction cannot be attained.

The amounts of impurities that may be permitted in a system vary with a number of factors, such as the specific geometry of the system and the form in which `the uranium isv used-that is, whether natural or en lriched with ssionable isotopes, whether as metal or oxide-and also factors such as the weightratios between the uranium and the slowing down material, and the type of slowing down or moderating material used-forexample, whether deuterium, graphite or beryllium. Al-

Vthough all of these considerations influence the actual 'permissible amount of each impurity material, vin general ythe effect of any given impurity or impurities canv be correlated directly 'with the weight of the impurity present and with the K factor of the system, so that knowing ithat a neutron chain reaction can be obtained.

the K factor for a given geometry and composition, the permissible amounts of particular impurities can be read-- ily computed without taking individual account of the specific considerations named above. Different impu rities are found to affect the operation to widely differ-- ent extents; for example, relatively considerable quan-- tities of elements such as hydrogen may be present, and the uranium may be in the form of oxide, such as U02 or U3O8, Vor the carbide, although the metal is preferred. Nitrogen may be present to some extent and its effect on the chain reaction is such that the neutron reproduction ratio of the system may be changed by changes in atmospheric pressure. This effect may be eliminated by enclosing or evacuating the system if desired.

When the uranium and the moderator are of such purity, and the uranium and the moderator are so combined that fewer neutrons are parasitically absorbed than are gained by fission, the uranium will support a chain reaction, producing an exponential rise in neutron density, if the overall size of the system is sutlciently large to prevent excessive loss of neutrons from the exterior of the system. Thus the overall size is important..

The overall size of the system will vary, depending` upon the K constant of the system and upon other things: including the type of moderator used. If the multiplication constant K is greater than unity, the number of neutrons present will increase exponentially and indefinitely, provided the structure is made sufiiciently large. If, on: the contrary, the structure is small, with a large surfaccto-volume ratio, there will be a rate of lose of neutrons. from the structure by leakage through the outer sur-- faces, which may overbalance the rate of neutron production inside the structure so that a chain reaction will not be self-maintaining. For each value of the multi-l plication constant K greater than unity, there is thus4 a minimum overall size of a given structure known as. the critical size, above which the rate of loss of neutrons'. by diffusion to the walls of the structure and leakage away from the structure is less than the rate of produc-- tion of neutrons within the system, thus making the chain reaction capable of self-maintenance. The rate of diffusion of neutrons away from a large structure, in which they are being created, through the exterior surface thereof may be treated by mathematical analysis when the value of K and certain other constants are known, as the ratio of the exterior surface to the volume becomes less as the structure is enlarged.

vHaving thus generally dened the losses encountered in a self-sustaining chain reacting system operating by virtue of nuclear ssion by thermal neutrons, it is a further object of our invention to provide a means and method of correlating the neutron losses in a system of practical size comprising uranium and an ecient moderator to obtain a self-sustaining chain reaction capable of producing power, with full control over the power output of the system.

Chain reacting systems operating by virtue of nuclear fission may be classed in various categories, in accordance with the form and shape of the uranium bodies and the type of moderator utilized.

For example, when either beryllium or carbon in the form of graphite is used as a moderator, surface resonance losses are reduced to a point Where K factors up to at least 1.09 can be obtained by aggregating the uranium into spheres, cylinders, rods or approximate shapes, geometrically disposed in at least one plane in the moderator. The optimum K constant is obtained for uranium metal spheres, decreasing as the surface-volume ratio of the uranium bodies is increased due to increase of surface resonance absorption. Likewise the optimum K constant is decreased by the use of uranium in effectively less dense form than the metal, such as the carbide, or the oxides U02 and U3O8. For every-K constant above unity the overall critical size of the structure is such The .=.,f9110w1is.fspeeic .examples willl indicate-,tha approximate characteristics of graphite-uranium systemsusingmatetyrials of presentlyavailable purity.

:.When'aDO heavy water-.or deuterium oxide)=is` used' .as a. moderator, :the system, .due -tothe etlciencyofv the D20 as a moderatonis capable'of operation as a chain reacting system even when the uranium is in the form @ofi-tine:.particlesr powder suchas U02 dispersed as a .a'slurry.- in:D20,-.where the -particles .are of from a fraction :ofgafmicron to 'about` 50 microns in: diameter. When a U to D2 ratio of .0025 to 0.040 is used and--thefslurry :istplace'd in,.-for example, a spherical tank 13.4f ft. to -56.5ift..=in, radius, the chain reaction will be maintained.

. :The use.of theslurry will greatly incre'asevthe surface resonance absorption, but as the reaction whenDO is -used' willtakefplace in spite of the increased absorption,-l

it follows that K factors up to 1.3 can be obtainedby '.reducingfthe-.resonance absorption by usingV the uranium in the form of bodies of substantial size dispersed -in 1320. vIfhehighgKfactors and the favorable characterfisticsgof: .the;deuterium permit the overall sizeof such systems-to bev much smallerthan when graphite or beryllium are USedasmQderatOrS. For example, the minimum Uamoujntfoffgheavy water required for sustaining a chain .fr eactiorrgfor--optimum uranium rod geometry isV about 4 tons Vofheavy 'water with 2.1 tons of uranium rods dispersed therein. :A self sustaining reaction may be provided with 136 uranium metal rods 4.1 ft. in length and 1.1 vinches in -diameter spaced 5.5 inches center to center and having an aluminum sheath .035 nchesthick when a tank lled with heavy Water enclosing the rods -is used, the tank being surrounded by a layer of graphite .approximately twofeet in thickness to serve as a neutron reflector.

The fact that neutron absorption losses in excess of inherentlosses Vmay be introduced in these chain reacting -systems leads to eective control of the chain reaction. TBy building thesystem to have the ability to develop a 'chain reaction, losses may be deliberately increased to maintain the chain reaction in balance at a predetermined neutron-density, and consequently at a predetermined power output in the form of heat.

fixIn -the systems having solid moderators, it is conivenientto control ythe reaction by introducing neutron absorbing impuritiesvinto the system. When a liquid moderator vsuch as D20 is utilized, the reaction can be -controlled by introducing impurities into the system or by otherwise varying the critical size, or by varying the eective size (as by varying thefamount of moderator in the tank),or by combining various methods.

The'efect of impurities on the multiplication Vconstant Kfof any system may be conveniently evaluated to a reasonable approximation, simply by'means of certain constants known as danger coefficients which are assigned to the various elements. These danger -coeicientstorftheimpurities areeach multiplied by the per- .cent byweightfof the corresponding impurity, and the totalsumof these products gives a value known as the ltotaldanger sum. This total danger sum is subtracted ,from the multiplication constant K as calculated Yfor pure fmaterials and v for the specific geometry `under consideration.

;-The,danger coefficients are defined in termsfof the ratio of the weight `of impurity per-unit mass of uranium and .farei-based on thecross-section for 'absorption (of thermal ..nsat10ns, of;the yaIiQuseIementS. yThese .values maybe @brainedritorniphysissstsstbopkspnft efsubiectrandfthe e daneerecoecientf cemputedffby. the f mula wherein irepresentsftlie 'crss-sectiofor the impurity and wille .cross-Sestini:for.ftheveertien-Arme atomic weight of the `impurity andaA,the-atomicweight-of uranium f IIf theimpurities are inthe, v-.carfbom y1 theyfare computedas their percent ,1.0i the weight of-the ,uranium of the system.

Danger'coeticients -for s o me1 elements are givenfini the following table, wherein Ithe .elements are assumedfto;have

V4tl1eir-natural :isotopic constitution unless otherwise indicated and are-A conveniently listed .-according to @their chemical symbols:

- Dan er .Diu' r Element: .coetiic ent .f Element: coettleent :H1 1.0 D2

':` He i 0 j'fRh 50 I Li 3 10 Pd Y e2 i" -B y2,150 r cdV fs-70 l C 0.0 12 In v 54.2 "IN^ 1 5SH- .0 0.002 .Sb 1.6 F I0.02 Te 1 Ne v' 3 l I 1 y1.6 :Na f 0.6'5 Xe y 6 Mg 0.48 Cs 1 8.7 A1 0.30 Ba 0.30 f Si f 0.26 -La 24 3' 0.3 Ce S 0.46 Pr 2.4 Cl l 31 Nd y-17 ',A -0.8 -Sm ,-143O yK 2.1 VEu 435 Ca 0.37 Gd -6320 Sc 7 Tb -20 Ti 3.8 Dy A200 V 4 `Ho -10 Cr 2 Er N40 Mn 7.5 Tm H20 Fe 1.5 Yb m10 Co 17 Lu 30 Ni 3 Hf 20 Cu 1.8 Ta .1.4.6 Zn 0.61 YW f2.7

vGa -1 `Re 18 Ge 5) Os 'f '1.7 As 2 Ir 70 Se 6.3 Pt -2.5

, Br 2.5 Au 's-16 Kr 6 Hg i r82 Rb -0.4 Tl 0.5

Sr 0.57 Pb 0:03 Y 0.4 Bi 050025 Zr -0.13 Th r1.1

0f course, where an element is necessarily ,used inan active part of a system, it is `stillto vbe c( Jfn's'iderfed as 'an impurity; for example, in a "structure Where the uranium bodies consist of uranium oxide, the actual K ,constant would ordinarily be computed by. tak ing that factinto account, using ,as a base'K a value computedfor theoretically pure uranium.

As a specic example, if the materi als o f th'esystem under consideration have .0001 part by weightof H, I Co, and Ag, the total danger sum in K units for Vsuchran analysis would be:

.oo01 1o+.0001 17;.00o1 18:.0045r;I algas This-:would be 'aI-rather unimportairtfredu'ctibninithe reproduction constant K unless the reproduction constant -the reproduction ratio.

4for a'givensystem, without considering impurities, is very nearly unity. If, on the. other hand, the impurities in the uranium in the previous 'example had been Li, Co, and Rh, the total danger sum would be:

` This latter reduction in the reproduction constant for a given system might be serious and might well reduce the reproduction constant below unity for certain geometries so as to make it impossible to effect a self-maintaining chain reaction with natural uraniumand graphite, but might still be permissible when using uranium in a D20 system having a high K constant.

This strong absorbing action of some elements renders a self-sustaining chain reacting system capable of control. By introducing neutron absorbing elements in the form `of-rods or sheets into the interior of the system, for

instance in the moderator between the uraniuin masses, -the neutron reproduction ratio of the system can be changed in accordance with the amountv of absorbing material exposed to the neutrons in the system.V A sufiicient mass ofthe absorbing material can readily be inserted into the system to reduce the reproduction ratio of the system to less than unity and thus stop the reaction.

Where D20 or other liquid is used as a moderator, the reproduction ratio can be efectively changed by changing the volume of the system, that is, by pumping out and pumping in D20 to vary the volume of immersed uranium from below to various values above, critical size. The greater the volume above critical size, the greater will be As the volume is increased, the neutron leakage losses through the outer surfaces decrease.

A still further object of our invention is to control a self-sustaining chain reacting system by varying the losses in the system.

The foregoing constitute sorne of the principal objects and advantages of the present invention, others of which will become apparent from the following description read in conjunction with the drawings, in which:

Fig. 1 is a diagrammatic representation of the neutron utilization in a self-sustaining chain reaction process;

Fig. 2 is a diagrammatic showing of the various control features (exclusive of circuits) appliedV to a self-sustaining chain reacting system comprising titanium lumps inter spersed in a graphite moderator;

Fig. 3 is a modification of Fig. 2 showing a removable Stringer, instead of a regulating rod for system control;

Fig. 4 is another modification of Fig. 2 wherein the chain reacting system comprises uranium rods immersed in a deuterium oxide (heavy water) neutron slowing medium;

Fig. 5 is a schematic diagrammatic showing of the control system, exclusive of the indicating system, for performing the control features illustrated in Figs. 2, 3, and 4;

Fig. 6 is a plan view of the safety rod operating mechanism;

Fig. 7 is a view in side elevation of the structure shown in Fig. 6;

Fig. 8 is an enlarged side view, partly in section, of an electromagnetically operated catch associated with the safety rod in Fig. 6;

Fig. 9 is an enlarged sectional View taken along line 9--9 of Fig. 6;

Fig. l is an enlarged plan view of a modiiied form of safety rod operating mechanism illustrated in Fig.

Fig. 11 is a side view of the structure shown in Fig. l0;

Fig. l2 is a plan view of the regulating or control rod operating mechanism illustrated in Fig. 5;

Fig. 13 is a side View of the mechanism illustrated in Fig. 12;

Fig. 14 is an enlarged sectional view taken along line 14--14 of Fig. 12;

- Fig. l5 is an enlarged sectional View taken along line 15-15 of Fig. 12;,

Fig. 16 is an enlarged sectional view taken along line 16-16 of Fig. 12;

Fig. 17 is a schematic showing of the indicating system embodied in the control system shown in Fig. 5;

Fig. 18 is a schematic showing of a modification of the safety rod operating mechanism shown in Figs. 6 to 9, inclusive; and

Fig. 19 is a chart showing time for neutrons to double versus reproduction ratio in a chain reacting system.

To illustrate the importance of the various factors enterirnT into a chain reaction, an example of a chain reaction process will be described, as it is presently understood to occur in any system of nite size utilizing, for example, natural uranium dispersed in graphite, deuterium, or other neutron moderator.

Referring to Fig. 1:

A represents a uranium body of any size from which fast neutrons are set free as a result of the ssion process.

B represents a fast neutron loss due to leakage from the system.

C represents a uranium body of any size in Which both volume and surface resonance absorption of neutrons by U238 takes place at resonance energies above thermal energy to form 94239.

D represents the number of neutrons reaching thermal energy.

E represents a thermal neutron loss by diffusion of thermal neutrons from the system.

F represents a neutron loss caused by capture of neutrons by impurities'in both uranium and neutron slowing material as well as neutron loss caused by insertion of neutron absorbing material into the system.

G represents a neutron loss due to capture of thermal neutrons by the neutron moderator as the thermal neutrons diffuse therethrough before entering uranium.

H represents the number of thermal neutrons entering a uranium body.

l represents a uranium body of any size in which part of the thermal neutrons entering the body are absorbed by U238 to form 94239, the remaining thermal neutrons causing new iissions, thereby producing new fast neutrons.

The four neutron losses from the chain reaction referred to above are represented in Fig. l where the resonance absorption at C and the fraction of thermal neutrons absorbed by U238 at l represent the uranium absorption losses. Losses due to impurities are represented at F, those due to absorption in the moderator at G, and the leakage losses due to the iinite size of the system, at B and E.

For purposes of control, the losses due to impurities, by insertion into the system of materials which absorb a substantial amount of neutrons such as cadmium, boron, etc., are of outstanding importance, since such losses more readily lend themselves to variation.

By using neutron absorbing control rods, the system may be controlled automatically as well as manually. Since the power produced by a chain reacting system is proportional to the rate of generation of neutrons in the system, it will be readily apparent that the control may be made responsive to the neutron density in a representative portion of the system, such as a portion within and adjacent the outer walls-or even exterior of the outer walls. From a knowledge of the neutron density distribution within the systemy (generally a cosine curve distribution with maximum density at the center) the neutron density in any part of thesystem is readily determined. A convenient device for measuring or detecting such neutron density is an ionization chamber having electrodes including a collecting electrode inserted in a medium which can be ionized as a result of neutron bombardment, s-uch as BFB, argon, Freon, or the like. Such ionization chambers are well known in the art and, per se, form no part of the present invention.

It is generally desired to maintain the neutron density Aof the system at a substantially constantA predetermined .petasse ..value,.ialthough.while starting the-system 4from apoint where the reproduction ratio is appreciably .below unity when all the neutron absorbing rods are inserted therein, it may be desirable to bring the .system to a reproduction ratio of above unity in steps taking minutes, or perhaps hours or days. The operation of the control system will -'belbetterunderstood bythe following description'of types "fof controlused andthe vvarious operations which take place as the neutron absorbing rods are' inserted into and 'withdrawntfrom Vthe system.

FR'eferringrmore-particularly to' Fig. 2, numeral 1 denotes. ablock 'diagram-'that represents a self-sustaining -neutron chain reacting system such =as,fforfeXample, one

havinga=lplurality 'of lumps Hof uranium containingmate- 'rifal (or uranium) interspersed*geometrically in the form ofrlatticefstructurein a neutron slowing medium such las graphite, :preferably 4vinV the.fo`rm 'ofI a 'plurality of closely stacked blocks. For this reason system 1 is valso -refer-red.tor'as a' pile. -Nurneral..2.Jdenotes,ij generally, a ffsafety'mechanism':comprising a safety rod incorporating. -ai'materialwhlch.lcanabsorb a substantialnumberof #neutrongsuch a's boron, cadmium, gadolinium, samarium, etc. T Thesafety rod-i`s= normallyfhe'ldy in a withdrawn or tkretrievedpositionfrom. `system 1 =by=rneans of' a -catch lheldengagedeinav perforated portionvof thegs'afety'srod'y operating through a pulley system to pull the safety'r'od 1 into 'lthe '-systemfso as to bring "the neutron 'reproduction Yfaction. Theapurpose of the isafety .mechanis'misrto proratio'fbelow unity and'stop .the self-sustainingchainirevid'e 'emergency'rneans for stoppingfthe.reactionfin .the .system if some abnormal condition 'should arise, such was an excessive: rlse'in neutron density. Theinechanism Willabei'describedin :detail hereinafter with referenceto Fig-5.

:fNumeral ffl-.denotes a .regulatingor'controlmechanism "'such' as vai-pair of'mechanically 'coupledfmotorg 'one'for systemvinresponsewto anincrease :or decrease-of neutron 45" -driving a regulatingrod ofzneutron` absorbingfmaterial into the system and the other for drivingl it out ofzthe idensity, respectively, as'indicatedl `by' ther .neutron density responsiveidevice 3, ksosas to maintain the neutrondensity "at a "substantiallyconstant value iasv will be idescribedtin ldetailhereinafter, in connection WitheEig'. 5

#Numeral 5 denotes a limiting mechanism comprising alimitin'grod; or a pluralityof rods, :of neutron absorbingmaterial .which can .be projected-into thel system lany desira'ble 'amounttand locked. inI place by a tapered- -pin land'aapadlockor equivalent locking device if so desired.

Thepnrposeof l.the ilimiting rod is to limit the neutron reproduction ratio lof .the system to a predetermined .maxif;rnum:value..1;lt'may also be used to compensatey for a .change:inltheeproduction ratio of the system caused by a changein temperature, a change inthe amount .of im- Afpurities in the* system Vresulting from operation and to .c'onipensate-ffor an increase -in the reproduction ratio caused byenrichment of the uranium through' the fpro- -"ductionzofrthe iissionable nucleus 94as the resultofcony,pensatedfor the dierence in` reproduction ratio caused'l Yt'nued 'operation'of the system.. lt is also used to com- 'by increase in temperature when the i systemy is' started .from a coldcondition and is broughtup to a Warm-.or

thotc'perating condition. The rate of neutronfrise Within "92235. Since someof the neutrons, approximately lpercent of the fission neutrons, 'are delayed in their -emerl4`Sion it is generallydesirabletofkcep the-maximum attainf able; reproductionratiobelow 1.01, that is, withimthe -rangeof `th'e;.delayed neutron .eiect-as explained more iffully hereinafter.

Fig. 3 shows a modification of the device Yshown in .'Fig. 2 and has'iden'tical parts which are indicated by vsulixingthe yletter ato'the reference numerals usedj-in Fig. 2. Fig. 3 differsfrom Fig. 2 only in the regulating mechanismaeta which comprises a Stringer 6, such Stringer 6 being in reality, an elemental portion of-s-ystem 1a, that is,;,a-p,ortion having the samedimensional structure ,of uraniumcontaining materialin a graphite mediumfas the rest of 1a instead'ofbeing fmerelyga-Iod .ofmeutron fabsorbingrmaterial asyshown -in- Fig. 2. .The purpose'pff the regulatingmechanism 4a is the. sameg'as that .of twin` Fig., 1,1`namely, to, changefithe reproduction i ratio in lthe :system vby :changing thev space relationship 'of'. one: portion. oftheisystem tothe remainder. Eorigexample, the Stringer: 6 may comprise a section.L of thexreactor-'that ise'movagbleV into. and outof they active-,portion of-the system, theA .reproduction ratio increasing ;as; the .'str-inger `isrinserted therein', and decreasing upon removal. 'Ther-Stringer.mayfpreferablyl comprise uranium lumps; dis- -tributeddn anf elongatedwsection =of rgraphite extending 'in a direction aligned-withfthemechanism 4a, the uranium ./bein'g Athe "same spatial relationship as v-inithe remaining J'section oftheiact'iveportion ofthe reactor.

'z Fig; 4'shows a further modification of the device shown 'in Fig..v Zmand fthe yvario'us mechanisms having the same .purposetarerfdenoted'by thesame reference numerals'ex- 1 ceptfthat a sux bis added; for example,- system 1b *of Fig. 4'corres'pond's tosystem 1 vof Fig.; 2, etc. Instead @f1-'using wuranium containing rmaterial :in a graphite medium` as shovyn'inyFig. 2, thesystemy1b comprisesxa plurality-.of parallel :rods of uranium containing Amaterial immersed in a neutron slowing material of deuteniumuoxaide. dhersafetyfmechanismi Zb. and 'theglimiting' mech- ;:ahism' 5bl are; substantiallyiidentical nto; 2;.and;5,:respec .--tive1y,-:infFig.; 2. Thewregulating' mechanism 4b;:differs from. 4-in-Fig 2-gin that the twormotors useditoydrive v inwopposite directions-havethe function; of;-rdrivir1g, afreyersible -pump in opposite directions to fpump. more` heavy Ywatereitherinto ythe system Vor -out of the tsystemfso, as f to -increase..the :volume yabove critical fsize orfto; bri-ng fit below, critical f size, depending -on whetherl fthe nnoulon density. indicatedA -by;=indicator 3b: is,;below; .or;jab.ive;a `predetermineddesired value. The vultimate1 purposesof -mechanism'Abin lFig. 4V is the Lsame asuthat-fof. @ginfig 2;` namely, to alter' the neutron reproduction .-ratio. -v -In rsteadfof -usingxthe mechanism 4b to vary. thefreproduction .ratio fby variation -of the heavywater leveL-thecontrol rod structure 4..of Fig. 2 may be used. fllhusathe-level of heavy Watermayl be initially adjustedfwith 4the control rod insertedto a height'such .that the reproductiongratio is greater .than unity upon. partial-removal of-they 'control rod. :Hou/ever, .in a liquid moderated system the yfeatures shown in Eig.. 4 utilized for controlling the maximum reproduction-ratio. may ,comprise the ylimiting .mechanism .5b.y ,Whichriricludes a plurality of Yoverllovy.y ports loi' va1vs, 'any one ;of which' maybe. selectively lopenedto maintain the level oftheheavy Waterat apredetremined .maximum value, thereby having rthe sarnngeneralzuiosetfas limiting mechanism 5 of Fig. 2,' namely, that of liinltlng the neutron reproduction ratio to a predetermined maximum permissible Value androf affording adjustment of such maximum value.

While lumping of the uranium is indispensable in systems using graphiteas a slowing medium or moderator such as described in vFigs; 2- and 3, it is not very important in thestructure shown in Fig. 4 usingheavy .water since *the .absorption lossestherein vare a'lmost v.111'egligible. YA modicati'on ofthe structure, shown inFg. `4,.1v'vherein our control jsystem is equally applicable, is a slurry system having nely divided fssionable material such 's particles v4of U02` 'suspended -in heavy water pumped through tubesand surrounded'byl a heavy water slowing -rnaterial (i: efrnbder'atorl In-fact, the vactive p'tidn Y may comprise a tank containing a Vmass ofV such slurry externally circulated for purposes of cooling and our invention is applicable to any chain reacting system and is not limited to those described.

A general description of certain control characteristics and method of controlling the reactor will now be given and will be described more in detail later with reference to the operation of the control system illustrated in Fig. 5.

After the neutron absorbing rods are withdrawn from System 1 and, in order to stabilize the reaction at any desired neutron density within the system, the neutron density is measured as it is rising. When a predetermined neutron density is reached within the system, the cadmium or other neutron absorbing material is reinserted into the system to a point where suicient neutrons are absorbed to prevent an increase in the neutron production, the absorption being by reason of the impurities introduced into the system. The chain reaction will then be in balance at the new neutron density. To reduce the neutron density, still more absorbing material is introduced into the system, suicient, for example, to increase the total impurity absorption to a point suicient to absorb additional neutrons. Under these conditions less than the necessary amount of thermal neutrons for a self-sustaining chain reaction will be absorbed in the uranium and the neutron density will decay, because fewer neutrons will be produced than the number of original neutrons in each successive chain. The system can then be stabilized when the new desired lower density is reached by partially withdrawing the control rod, thereby decreasing the neutron absorbing material in the system until the number of neutrons developed are sutlicient to maintain the new lower neutron density.

As indicated above, an important characteristic in the control of the pile is the fact that not all of the fast neutrons originating in the uranium leave the uranium immediately. About 1 percent of the fast neutrons are delayed neutrons. These delayed neutrons may appear at any time up to several minutes after the fission has occurred. Half of these neutrons are emitted within six seconds and .9 within 45 seconds. The mean time of delayed emission is about 5 seconds; The reproduction cycle is completed by 99 percent of the neutrons in about .0015 seconds, but if the system is near the balanced condition the extra 1 percent may make all the difference between an increase or a decrease in the activity. The fact that the last neutron in the cycle is held back, as it were, imparts a slowness of response to the pile that would not be present if the originating neutrons at A (Fig. 1) were all emitted instantaneously.

For cases wherein the reproduction ratio (r) is above unity by appreciably less than 1 percent, the rise of neutron density, or more specifically the value n to which the number of neutrons has risen from an original value no, after a lapse of time -of t seconds during and before which the pile has operated at a fixed value of r (n being the number of neutrons at the beginning of t, i. e., after disappearance of transient effects due to any preceding change in r), is given by and t istime. In this formula a is the fraction of the neutrons that are delayed, i. e., a=0.0067 and T is the mean time of delayed emission of the delayed neutrons, or seconds. The above formula is only approximate because it uses an average delay time.

As an example, assume'as a result of moving the convtrol rod, ,r becomes 1.001 and assume that the system has settled down to a steady exponential rise in neutron density. Then that is, n/n0=2.72 in 28.5 seconds. Thus doubling of the neutron density occurs about every 20 seconds. The above formula thus indicates the rate `of rise for relatively low values of r and shows how the reduction of the rate by the delayed neutron eect is particularly significant in the stated lower range of r values. Strictly speaking, the given equation holds only for the steady state, i. e., where r has been held constant for some time; an additional transient term must be included to obtain an accurate representation of the neutron density during the first few seconds after a sudden change of r. See Fig. 19, also.

If r were made exactly 1.01, a more detailed theory shows that the neutron density would be tripled each second. However, if the reproduction ratio r is several percent greater than unity, so that one percent delayed neutr-ons are unimportant compared with r-l, the density increases at a much more rapid rate as given approximately by rt/l where l is .0015 seconds, the normal time for the neutrons to complete a reproduction cycle. Thus if r were to be made 1.04 the neutron density would increase in 1.5 seconds by a factor of approximately 101" over its original level. However, if r were 1.02 or 1.03, the factors by which the neutron density would be multiplied each second would be 1100 and 700,000, respectively.

It is thus apparent that too high a reproduction ratio in a practical system leads to the necessity of inserting what may be considered as an excessive amount of controlling absorbers. An exceedingly dangerous condition could exist if by accident these absorbers were suddenly completely removed, as the time required for reinserting the absorbing material might be too long to prevent destruction of the system. As the same eventual density can be obtained with a reproduction ratio only slightly over unity, as with a higher ratio, only at slower rate, the lower reproduction ratios are preferred in practice in the interest of safety.

It should be pointed out here that neutron absorbers inserted into the pile cannot continue to absorb neutrons indefinitely. The continued absorption of neutrons by the absorbing material causes transmutation of the absorbing material, and an element or isotope may be built up within the controlling material which has a smaller neutron capture cross-section than the original material. This action will occur in piles operated at high neutron densities, and is corrected by systematic replacement of the absorbing material at predetermined intervals. In this respect, it is to be noted that the safety rods provided are not normally fully inserted in the pile until the pile is shut down, and therefore are not subjected to prolonged operation at high neutron densities. Thus, they retain their effectiveness for emergency operation.

After the structure has been completed, it is ready for operation, utilizing the neutron absorbing type of control described, with reference to Fig. 2. The neutron absorbing safety rod 2 is withdrawn from the active portion of the system 1, and with the limiting rod 5 fully inserted, the neutron absorbing control or regulating rod 4 is then slowly retracted until a galvanometer (not shown) connected with the ion chamber 3 indicates that the neutron density is rising. Should the neutron density fail to rise, the control rod 4 is reinserted and the limiting rod 5 withdrawn a slight amount, such as by one of the tapered pin position lengths. The control rod 4 is then slowly withdrawn and this sequence followed until the neutron density begins to rise by exceeding unity position of the control rod. If a slow rise is desired, the rod is retracted only just enough to indicate a rise. If a faster of control of the pile may be manual.

v v 13 rise is desired, the-rod isretracted'further toincr'ease the reprodction'ratio in the system,

When vany vdesired neutron densityisfreached, the control rod is pushed back into the 'pile u til a'pontis reached at which the'neutr'on density rem'ainsconstant. At'this point the system is'balailcedywithy a `ne'utro'ri reproduction 'ratio of unity. 'No "speciali-source of rneutrons `is needed in the structure, asxthe'naturalneutons v'always present and constantly diffusing 'through 'the lpile-are sucient to start the reaction.

To again 'increase the neutron density, the control rod 4 is moved 'outwardly' in such a"mannerand to such extent that neutron density rises'lat a desired "rate and attains the` new 'desiredV value and then thecontrol -rod is moved into the system-to Ithepoint" where the system` neutron density'in the pile, but onlythe `rate `of change ofthe density.

The unity reproduction ratio position of the control rod within the pile lfor maintaining any desired neutron density would always be the same were it not for thc' l fact that the temperature within thepile changes to some extent and influences the neutronv losses in the materials and also for the fact that in anypile exposed to atmospheric pressure, changes in the nitrogen content of the pile, accompanying `changes inl atmospheric pressure,i

'change Athe K factor since nitrogen is Aan absorbing impurity. The unity reproduction ratio position of the control rod therefore `changes slightly in accordance with the temperature at which the pile is being :.oper'atedland with the atmospheric pressure. Changes due to atmos-l pheric pressureA are small ybut are of such magnitudethat the position ofthe control rodk can, ifdesired, be utilized to measure atmospheric pressure,to the extent that the nitrogenconcentration in the pile can be-` taken as a measure of atmospheric pressure, as for'instance `when air circulation or other conditions are such that any oxidation reactions inthe pile do not significantly change the constitution of the air therein. An exceptionally accurate barometric measurement can thus be obtained.

If, at any time, it is desired to 4stop the reaction, the control rod is inserted deep within Vthe pile until the reproduction ratio becomes less than the value necessary to maintain the reaction with this sizeof structure. -The Vneutron density then decays to that of the natural neu-l trons. Thesafety rods are also inserted when thefpile is left unattended. The safety rods arepulled'into the systeml by weights tripped by an emergency latch in case of failure of the'control rod to control the reaction for any reason, as will be described in detail hereinafter.

An alternate method of control is to remove sufcient; uranium from the pile, preferably from pointsV close to the center, as illustrated in Fig. 3, to reduce the reproduction ratio to less than unity. This maybe done by the use ofthe removable Stringer 6 in the pile described,` and the movement of the control stringer would be re` versed with respect to the movements of the control rods: described fo-r control of the pile. In other respects, however, the action of a control Stringer would bethe same as the action of a control rod. The use of a control rod is preferred, however, as being simpler to operate and also because of radiation from the Stringer.

Itwill be apparent that the above described method While such man ual control may be satisfactory for systems having ob tainable reproduction ratios of only slightly oVer.unity Then the control L Ald such as 11.005, forexample, -fitwwill'not be satisfactory forfsystems havingfa*reproductiondratio:of a valuezapf preaching L01 in'- which-casesi the.delaye(l1-neutrons are far1less-etectivc inpreventing the-neutron production from risingl expone-ritirilly'-atl a fast frate. f A-.nyautornatic control system -is especiallyadesirable forHsuchsystems,

lespecially 'one whichis not only responsive Vtov thefchange ofneutron density Abut t'othe-rate of change-of-such density -in the system.

Fig. 5'sl1ows such anautomatic y.control isystem. elteferring to Fig. 5 'numeral-1, as'in Eig. `l, vdenotesfthe selfsustaining chain reacting ;fsyst'em corgp'ile,portions only 'of which yare shownjnumeral 2 denotes the safety `mechanisrn, fnumeralfS t'he'rneut'r'on' density responsive device,

and numeral 4` the regulating orcontrol mechanism.

The safetyrnechanism 2-'cornprises af'safety 'rod '7 ofy neutron absorbing materlalf'such'asffcadrniurnpborom "ete, .or of "material including substantial'amounts `of'su'ch neutron absorbing materials.` 5 The safetyrod 7 is vnormally held withdrawn, that is, .in aretrieved'fposition yWith'respect toxthepile by an letctromagnetically operated catch S which-engages a-chain 9y when-theaelectromagnetzcoil lil isenegized. The detailsA of' the electromagnetically-operated catch-are ybettens'hownin Fig; 8 whichf'willbe -descri-bed later. 'When-the current in `the coillt)l is inter ruptedeitherby manual interriuption of the energizing circuit, or automatic interruption fof said Acircuitf'as', the

' not absorb 'neutrons appreciably) :by gravity, .and :to 'pull withl it lthe `safety -rod' attachedlftheieto {intosthe pile? 1 to stop the neutron chain reaction. While only' one safety rod 7 is shown, it will be apparent that a plurality of such rods may be `operated simultaneously. As the safety rod is pulled into the'pile, a cam 1 4 integrally secured' :thereto'flwill allow 1 clsing :'of. flimit" switch :'15 to l 171is'e1osed which 'completes an energizing circuit-.from

a 115 volt alternating current power; and feederfconductor r51v through rewindniotor '16, ilwhich in turn willdrive oftherod'frorn the systemor pile 1. t"During thistir'ne catch 8 is in engagement with chain 9 becau'sec'oil'ltl is energized. This retrieving action will continue until cam 14 effects opening 'of limit switchrlS'at'the limit of travel of the safety rod.

' The regulating or control mechanism 4 V(Fig. 5) cornprises a regulating lrod y'18 of neutron absorbing'mate'rial, such; as cadmium,l boron, etc'., `which'likewise is insertable into the pile. Rod `li'has rigidly secured thereto a' rack 82 which engages with a pinion 19fdriven by a belt 20 either in one direction or another, depending upon whether motor 211er motor .2 2 is furnishing thernotivepower. Motors' '21"and '22"'which'may be'series motors, for example, are mechanically Vcoupled within the gear box 23, and together drive a pulley 24 either clockwise or counterclockwise. If motor21 is energized to a greater degree thanmotor22 it will drive rodA `18 into the pile if to a smaller degree, inotor v22 will drive it out of the pile. The use of two motors .rather than a` lsingle reversible motor is for better adaptation to an automatic regulating circuit to be described hereinafter.

However, motors 21 and`22 may be controlled manually by a Variac having a split winding, that is, windings 26 and 27. Now, for example, when switch 57 is in its lower position and switch 101 and 102 are in the open position indicated in the drawing,the` system isset for manual control through movement of the Variac knob 28...',If the knob :w28 is moved clockwise and in reacting relationshipy withwinding 27 the energization of motor 21 is increased and the regulating rod 18 is driven into the pile,

For this reason, the motor 21 may be termed the in motor. If the knob 28 is turned counter-clockwise and in reacting relationship with the winding 26, motor 22 is energized so as to cause outward movement of the regulating rod. For this reason, motor 22 may be termed the out motor. The speed at which motors 21 and 22, operate under such a manual con-trol system, thus depends upon the setting of the knob 28 with relation to the windings 27 and 26 respectively. Under the manual control system, it is then clear that when one motor is energized the other is idle and is driven backward (i. e. in rotation) by the energized one. As will be more fully explained later, when the system is controlled automatically, the motors 21 and 22 are both energized, the rotation of the pulley 24 is determined by the relative energization of the motors, that is, if motor 21 is energized more than motor 22 the regulating rod 18 will be driven into the pile and the speed at which this action takes place depends on the relative energization of the two motors as imparted by the automatic control system.

Before describing the automatic features of the control system, it may be well to summarize certain general principles of such control. Firstly, the power developed in the system may be expressed in terms of neutron density in any portion of the system as measured. Since the temperature of the pile varies with neutron density, the controlling impulses may likewise be given by a temperature responsive device (not shown) such as thermocouples or the like. However, for our purposes, because of temperature lag, we prefer a neutron density responsive device such as an ion chamber.

When a neutronic reactor is controlled by neutron density, the following assumption is made, as an approximation:

dt T

where dri/dt is the rate of change of neutron density, x is the distance of the control rod from a neutral position at which the neutron density remains constant, and K1 is ay constant (not to be confused with the reproduction factor), lthe direction outward from the neutral position being considered as positive.

Suppose, then, that the control system is designed to move the rod at a speed partly proportional to the neutron,

dix 2n du duJfKuiKu-O Substituting from the first equation, we obtain:

dix

f da: diz This is now an equation of motion for the control rod alone and it is possible to insert any initial condition in terms of an initial error in rod position and solve for the motion followed by the rod in correcting for this dis-v turbance. It is found that if K3 is greater than there will be no oscillation of the control, i. e., the response will be damped; if K3 is equal to the stated eX- pression, there will be critical damping; and if K3 is less than the stated expression, therewill be oscillation which, though damped., will permit undesired hunting by the control. The foregoing assumes that K2 and K3 are both positive, as otherwise the control will be unstable.

In the present system, the neutron density responsive device 3 of Fig. 2 comprises, as best shown in Fig. 5, an ion chamber 31 having an internal collecting electrode 32, and an external electrode or chamber 33, the latter of which is grounded. Such chamber is well-known in the art. Positive ions developed in the chamber as a result of neutron bombardment of the gas contained therein will travel to the collecting electrode 32 and resulting positive charges will flow to the amplifier 34, shown in block dia- -gram form. By virtue of a battery 35 which is grounded, the amplifier is operated at a potential that is more negative than ground. The reason for doing this instead of using the more common type of amplifier operating at positive voltages is for convenience only, that is, the fact that it is more convenient to ground chamber 33 than to ground the electrode 32 of the ion chamber 31. By use of battery 36 in the output of the amplifier, the voltage is again made positive so as to provide a positive voltage impulse for the ensuing electronic circuit.

The output of the neutron density control device is passed to control mechanism 4 where the impulse or signal is amplified by vacuum tube T1 and then passes through a network consisting of resistors 37 and 38, and condenser 39. A characteristic of this network is that over the range of possible frequency components of any reasonable disturbance or impulse it applies to the grid of the next tube T2, a voltage given approximately by:

The constants of this equation are determined by the Value of resistors 37 and 38, and of condenser 39, and the latter may thus be used to adjust the entire control system for stable performances.

Reverting to the control circuit 26-28, the windings 26 and 27 each have in parallel therewith a transformer primary 29 and 30 of transformers Tr-1 and Tr-Z, respectively, upon closure of the manually controlled switches 101 and 102. Each of the transformers Tr-1 and Tr-2 has a secondary winding 40 and 41 respectively, across which two pairs of thyratron tubes 42 and 43 respectively, each pair forming a full wave rectifier, are so connected that when conducting, they shortcircuit the secondaries 40 and 41. The thyratron grid circuits are, however, arranged so that the amount of conduction, that is, the 4average conduction for any particular period, as for a half-cycle, may be continuously varied. This is accomplished by adding to the control potential, always supplied by tubes T2 and T4, an alternating current voltage of variable phase with respect to the thyratron plate voltage obtained from a transformer 44, operating through a phase shift network provided by variable resistor 45 and condenser 46. This adjustable alternating current is transmitted to the grids of thyratron tubes 42 and 43 through grid supply transformers 47 and 48 respectively. By varying resistor 45, thereby shifting the phase of the A. C. potential on the grid as compared to the phase of the A. C. potential on the plate of each respective thyratron tube, the point in time at which the thyratrons become conducting may be varied, thereby varying the period within any half cycle during which conduction occurs. In other words, the alternating current grid potential curve `superimposed on the direct current biasing voltage curve lobtainable from the output of tube T2 or T4 will give, at the point of intersection of such curves, the point in time at which conduction begins, and such point can be varied by varying resistor 4S of the phase shift network. Conduction ends generally at the end of the half cycle periods. The tubes of each pair alternately conduct current in a direction so as to provide continuous uni-directional current to the respective secondaries of the transformers Tr-l and Tr-2. Tube T3 produces an inversion of the control signal without amplification. Thus, changes 'in anode potential of T2 are accompanied by equal and opposite changes at the anode of T4.

For example, assume that the neutron density in the system 'has increased appreciably, thereby causing a substantial ion current to owV through the ion chamber. This current is amplified by amplifier 34 in a manner, as will now be apparent to those skilled in the art, such that a change of voltage in a positive direction is applied -to the grid of tube T1, the fixed bias on the grid of T1 being determinable by the difference in voltage of the batteries 35 and 36. This will cause a negative voltage at both terminals of resistor 37 and a corresponding positive voltage on the plate of tube T2. This positive voltage is applied to the respective grids of thyratron tubes 43. Since the grid of tube T3 is directly connected to that of tube T2 it will likewise be negative, causing both terminals of resistor 49 to be positive and causing the plate of tube T4 to be negative. The effect of the positive voltage on the grids of thyratron tubes 43 and the negative voltage on the grids of thyratron tubes 42 will be to make tubes d3 conductive for a longer portion of the respective half cycles than tubes 42, or in other words, to effect a more conductive shunt across secondary winding 41 than across secondary winding 40. This will cause a greater current flow through primary winding 30 than through primary winding 29 whereby more energizing current will flow through the inward driving motor 21 than through the outward driving motor 22, assuming the switches 101 and 102 are closed as indicated above for automatic operation. The net effect will be that the motors 21 and 22 will rotate in a direction to move the regulating rod 4 inwardly until the neutron density of the system has decreased suiciently so as to remove the positive voltage impulse from the grid of tube T1.

Similarly, if the density of the system decreased appreciably so as to apply a negative voltage impulse to the grid of tube T1 the output voltage of tube T2 will be negative and that of tube T., positive causing greater conductivity through thyratrons 42 than through 43, effecting more current flow through the energizing circuit of the outward driving motor 22 than through the inward driving motor 21, thus causing outward movement of the regulating rod 18 until a condition of balance has again been attained.

The control system is responsive not only to the change of potential applied to the grid of tube T1 but to the rate ot' change of such potential as well. The means responsive to such rate of change is the condenser 39 whose charging rate is dependent upon the rate of change of the potential imposed thereon. In this manner, the control system anticipates changes in neutron density, that is, it will cause a compensating control function appreciably before the change has fully taken place. In other words, if the neutron density should suddenly increase, the effect of condenser 39, responsive to the rate of change of the increase in neutron density, will be to control the potential on the grid of tube T2 in a manner so as to cause insertion of the regulating rod 1S into the system an appreciable time 'before the neutron density has been given an opportunity to rise to an abnormally high value.

While the system just above described may be used to drive a control rod in or out of a chain reacting system, it will be obvious that the same mechanism can be utilized to drive a pump in opposite directions, as required, to raise or lower the level of a liquid moderator above or below the critical size, as pointed out in the prior description of Fig. 4.

Various safety and interlock features are incorporated in the control system to effect control of the chain reacting system with a maximum amount of safety in the event of emergency, such as, for example, an abnormal rise in neutron density in the system or power main failure. Assume manually operated switch 50 is closed, thereby completing an energizing circuit extending from feeder conductor 51 through various automatic or manually operated safety relay switches 52 through the switch 50 and relay coil 53 to ground, thereby causing closure of 1S electromagnetically operated switch 54. This will set up a 'circuit extending from conductor 51 through switch 54, resistor 55 of high value, conductor 62, and coil 10 to ground. However, since resistor 5S is of high value, insufficient current will flow to effect attraction of armature or catch 8 by coil 10. However, as soon as the reset pushbutton 56 is closed thereby shunting the resistor 55, an appreciable current will flow through relay 10, there- 'by causing attraction of the armature or catch 8, which in turn will engage chain 9 and make it possible for the rewind circuit, including switch 17, limit switch 15 and driving motor 16, to withdraw safety rod 7 from the pile. In other words, unless switch S0 is closed and unless reset switch 56 is depressed it will be impossible to effect a rewind, that is, a withdrawal of safety rods from the pile.

Another safety feature is involved in the switch 57 which bridges the lower contact only when all the safety switches 52 are closed, so as to complete an energizing circuit extending through conductor 51 through safety switches 52, coil 58 to ground. When switch 57 is electromagnetically held against the lower contact, a circuit is completed extending from conductor 51 through transformer primary winding 30 to the inward driving motor 21 thence to limit switch 104 to ground so that the motor is controllable either by the electronic automatic control system or by the manual control system including winding 26 and 27. The energizing circuit is paralleled by and also includes primary winding 29 and motor 22 connected in series therewith, such winding also being paralleled by the manually operated Varac winding 26. If the feeder line 51 should be interrupted at any point, say at one or more of the safety relay operated switches S2, this will effect de-energization of coil 58 and bridging of the upper contact of switch 57 and completion of a circuit extending from conductor 51 through switch S7 (upper contact), motor 21, limit switch 104 to ground. This means that if any interruption occurs that may impair the safety of the operation of the system the inward driving motor 21 will be connected directly across the line so that the efrect will be to directly energize and drive the regulating rod 18 inwardly into the pile to stop the reaction. In case of complete power failure it will be remembered that catch 8 will disengage from chain 9 so as to allow the weight 12 to pull the safety rod 7 into the pile or system to likewise stop the reaction.

Figs. 6 to 9 inclusive show the structural details of the safety rod operating mechanism. The safety rod 7 comprises strip of cadmium or cadmium containing steel 59 sandwiched between a steel strip 60 and a fiber strip 61. Secured above the ber strip is a third rail 62, for example, of brass, serving as an electrical conductor for the energizing circuit of coil 10. The composite safety rod comprising the various strips described hereinabove is notched along its sides to accommodate a plurality of rollers 63 which ride along the upper edges of a U-shaped flange or track 64 extending along the bottom of the safety rod as will be more clearly apparent from a study of Fig. 9. It will be seen, therefore, that the safety rod will travel along the track with a minimum amount of friction, thereby insuring movement thereof in a minimum of time, less than one second, for example, when the safety rod is released by the catch mechanism described below.

Fig. 8 shows the details of the electromagnetically operated catch 8 showing how it is operated by knee action through a toggle mechanism 65. The knee action is adjusted so that it is not quite on the zero position when holding, thus the pull of both spring 68 and the weight 12 tend to release the catch. lt will be apparent that when the coil 10 is energized, causing armature 66 to move to the right, carrying with it the knee of the toggle, it will cause lowering of catch member 8 onto one of the cross-links of chain 9 so as to eiect driving engagement between chain 9 and the entire electromagnetically operated mechanism which is.`- mounted" ontheend of the safety rod assemblyv7opposite'the pile 1. This mechanism is rigidly secured toV thetopl ofv--the rod 7 and moves therewith. When coil 10 isde energized, spring 6% will effect bending ofthe-toggle mechanism and lifting of catch 8 outof engagement with chain 9. Since it is desirable to apply av brakingA action to the safety rod whenV it is near they limlitof its movement inwardly into the pile, aV brake-69 is-ploaA vided comprising a pair of brake shoes 70 pivoted'to links 71 forming, in effect, a parallelogram linkage. Springs 72 are provided for normally biasing brake-shoes 70 `slightly to the right so that when the-corresponding friction shoes 73, rigidly secured tothesidestoft-hesafety rod, come into engagement with brake`r shoes 70 theywill eiiect a straightening-out -in avertic al directionV ofthe parallelograrny linkages, causingbindiug\- of brakeshoes 70 and friction shoes -73 thusretardingthe' motion of the safety rod to the left. However,- asl-,soon as-the-safety rod is moved to the. rightfunder thepowerf ofthe rewind motor 16 brake shoes- 70 -are-carrie dithere1 with, thuseffecting movementofthe'parallelogramlinlq agesto the right-which causes disengagementbetweerrj shfoes 70-and73. Y

Figs.l and 11 represent top and side-views ofa simplified form of the safety operating mechanism shown; in Figs. 6 to 9 inclusive, whereinA the parts.are-essen` tially similar with the exception of the type of brake used and the type of electromagnetically operated catch` employed. A pair of springs 74 have their right` end* terminals rigidly secured to a suitable supportingV struc-` ture, such as, for example, the pile, by suitable lbracketsl (not-shown), but have their left terminals securedto and bridged by cross bar 76 which is fadaptedf-to-sliderelative to its support members'75Ysecured-toythe-pi-leg When ahook member 77 secured tothe safety-rodl 7; comes into engagement with crossbar-76 near the limit` of y'movement of. the rod inwardly of the-pile, the springs,` 74v will yield and offer resistance to the continued movedV ment Vof the rod into the pile and therefore willj eect'- a braking action thereof. The electromagnetic-coi110j attracts an armature 66a secured to;ahookv 8in-which" engages an opening 78 in the safety rodwhenv coi l\ `-1t)4 is energized. When de-energized; av spring368aretractsgjv the hook. 8a from the opening so asto'allow freedom; of movement of the safety rod to theeleftl y bye-weiglitt- 112:

Figs.. 12 to 16 inclusive show thedetails of tlttefregu-- lating rod operating mechanism. The regulating rod 184V may, for example, comprise a strip-79 o fsteeljcontaing; ing-1.5 -percent boron (see Fig. 16) sandwiched between4 anlupper steel plate Sti, /ef thick, andja-lowersteel; strip 81, y" thick, the latter being rigidly-'secured to; steelh rack 82. The regulating rod is, supported'byga plurality of rollers 83 which ride on theguppergedgesoff aU-shaped track Se (see Fig.` On this tracktisfalsorotatably mounted a pinion 19 for applying motivepowerftothe rack 82. Rollers 83, whenewithinrtheipile, on shoulder members 85- (see Fig; 14).

Fig. 17 shows an indication circuit forgiving la`visual` or recorded indication of the positions of^the various rods at all. times. For example, the safety rod-positionis in;y dicated only at intervals by means, ofr'microswitches *8,65 alignedl along its track. ClosngOf aparticular micro; switch will effect illuminationv of va yparticular"indicator lamp 87 in circuit therewith energized through a trans-v former 88, such transformer being fed'by the'conductors` 39 energized by a suitable source of alternating-current; potential. The regulating rod position is moreaccurately indicated by selsyn units shown schematica-ily, omprisf inga. selsyn generator 99 coupled `to the regulating rol mechanism and connected'to selsyn motors`91fang i"9 2 "v operating 'a recording pin 93 anda dial indicat'u'94re:` arrangement it hasbeenff'oiii-tgl^ 0-05i11chespf1iytdef. An even higher degree of accuracy is posspectively. With such an possible to set rod positiontwithin siredvalue.

29. sible-by:increasing-morgen ratio vbetwen the selsynunits androdg'meclianism; Looationofthe regulating rodfin orrouteof thes ystenriisy indicated bythe in lamptgf and outt`lamp 1(165 whose, circuitsA toground are; com-V pleted thioughlimit-switches 197 and;108, respectively.

Thelimitingv rod position-similar to the safetyv rod is also^indicated1atvintervalsby micro switches 95, each of which is connectedtwithone of'a number of'indicator lamps96 energizedjgby theconductors-89 through transformer 97; The gmicroswitches Ahave a plurality of sleevelike; members sfcooperatingtherewith to accommodate a taperedpin' 99;A Thesleeve; like members are affixed with-1respect"to"thepileand the pin 99, which passes throughanv openingjn li'miting'rod 5, is selectivelyin-Y sertedf'inany gofgsuchgsleevelike members so as to cause lockingofthe limitingQlrod in any particular position horizontally. Asj an addedLsafety feature, ay hinged cover plate ltltljis provided *to enclose-the pin 99. Cover plate; 10D'fmay'be'lockedbya padlockltil so., asto avoid tainpering the 'settingofj'thelimiting rodby unauthorized' persons:

Fignltfisuaschematic'showinglof a modification of the safetyrodoperatingwmechanism `described in Figs.' 6 to9,` inclusiyfe, ^an d `isV in manyjrespects, simpler. The. gravity operated mechanism is` the same, ,the brake is of the type difsclosedin Figs l0.and l11, but the rod retrieving mechanisrn is somewhat different. A motor 107 drives va worm gear 108 which.injturn drivesy a wormwheel 109, the latlier beingmdirectly connectedtoone of the driving eiements oE1a n 1elect1'omagnetic clutch 116. The other clutch elementsY rigidlyjsecured Eto a pulley 111 of the gravityl operated safetypodinserting,mechanism.` A cable 1213, whiehj pass es over` pulleyv 111' is attached to the righth'andendofjrodj'anha weightY 121 is suspended from the freeend ofjthe cable to keep it taut, weight 121 being. le'ssjhan weighftjlZ'.'` A'source of alternating potential. is used,,rectiiiedbyfullf wave copper oxide rectier 112, tQ p`1jovi de` ldii-ect current` to thev coils of the electromagnetic. .cluftcltgY Areleaseswitch 113 is provided to interruptth'e energization of, the clutch.

Assume that the safety rod is inside the pile as shown ii;t177 i gk.1, 8,. Bydepressingapull-out switch 114, a circuit is, :cgmpletedjextenling from the upper terminal of the alteinatjng ,soyr through motor 107, limit switch 115, thence to..tliiewloyverjA terminal ofthealternating current source. A, I 'his w' j iet-a driveto the worm gearV systemy 10,5.- 1 Stand `c 1 11- ",n a direction to retrieve the safety. rod frbmttheopileg Ifljitl 4is desiredy to stop the retrieving movement @oi-...atNanytime immediately stop the reactionV inI the -the, rele'aseMswitch, 113,isopened,thereby nter.- ruptingfthenerg' ationoftheelectromagnetic clutch coils thereby unccruplihgwormn whee1,`.109 from pulley 111.. ThisLwilLallQwthe weight 12at the left to pull .the safety rod 'intoithersysternw Ifjtis desired to. pullv the safetyrod al'th 'way out of;`the.pi1e,vswitch 114 s kept closed ulltlafllmitl switcjhll "iliterruptsthe motor energizing` current,y Thfeyjodiwill b held in such outwardV positipfn. due. 1.0. .its` inability ,to .drivethe Worm. gear back- An.;iud i'catingcircuit 'is lalsozprovided. energized bra,-givoltlsnurce.whichjllmnates.the in. lamp 116: through:linitwitclrll'lwhen therod isy in the system., Afl .Qll-.'1m12..11-l$ illuminated b'ysaid source through. the,limit`switch111SLWhilrthe-rod is outside of the system.y A suitable cam is aflxed to safety rod 7, such as a camming. Surfacelzzlas,illustrated in Fig.A 18), ,is providedalong the underside` of the. rod for actuating the limit switch- 11,5V whenjhfe safety ,rod.is, retracted, thereby opening the PQWrsupp ly circuittq motor,107 and, simultaneously ClQSng-.thgcircljtof1321119118., Braking may beeiected alsmbt autamaticallv actnatinathe ,clutch by any suitable means lrithbyvnllwhenhtherod has movedtonear .the` limitlofit ilmardmoyementfofthe system. This.,pro. vide s extremgl smoothactiomn since all shock istaken uu ingtheirietiondrye.hetweenthe pullout cableaud. its pulley.'

21 It is therefore seen that we have provided an eflicient control systemfor a self-sustaining nuclear chain reacting system embodying both manual and automatic features and embodying various safety and interlock features to make the operation of the system not only safe under both usual and unusual conditions, but also fool-proof.

While the theory of the nuclear self-sustaining chain reacting system set forth herein is based on the best presently known experimental evidence, we do not wish to be bound thereby, since additional experimental data later discovered may modify the theory disclosed. Any such modiication or theory, however, will in no way affect the results obtained in the practice of the invention herein described and claimed.

While certain types of self-sustaining chain reaction systems have been described, illustrative of the application of our control system, it will be readily apparent that the control system may be used with other chain reacting systems with equal effectiveness. .It will also be apparent that the above described control system may be modified without departing from the spirit and scope of the invention described.

Furthermore, it should be noted thatl the embodiments of our invention described and illustrated in the drawing are merely exemplary and should not be considered as limiting the invention inasmuch as our teachings will readily suggest alternate or equivalent structures to those skilled in the art hence, we wish to be limited only insofar as set forth in the following claims.

We claim:

1. In combination with a self-sustaining neutron chain reactor, a safety system comprising a neutron absorbing means, means for transla-ting said neutron absorbing means along a path extending from the interior of the reactor to the exterior thereof comprising means exerting a force upon the neutron absorbing means directed Atoward the interior of the reactor, means for withdrawing and restraining said neutron absorbing means out of said reactor including a motor, an electromagnetic clutch connected between the motor and the force exerting means for coupling said motor to said neutron absorbing means, means to energize said clutch to keep said neutron absorbing means outside of -said reactor, and means to -deactuate the electromagnetic clutch, thereby resulting in the insertion of the neutron absorbing means into said reactor.

2. In combination with a self-sustaining neutron chain reactor, a safety system including a neutron absorbing means, means to translate said neutron absorbing means along a path extending from the interior of -the reactor to the exterior thereof including a weight and a pulley system having a cable connected to the weight and to the end of the neu-tron absorbing means confronting the reactor for moving said neutron absorbing means into said reactor, and a control means for withdrawing said neutron absorbing means from the reactor comprising a motor, an electromagnetic clutch, a worm gear coupled to said motor, a worm wheel coupled to -the driven side of said clutch, the driving side of the clutch being connected to the opposite end of the neutron absorbing means, and means connected to the clutch for controlling -the clutch whereby the neutron absorbing means may be withdrawn or released at will.

3. The apparatus recited in claim 2 together with a limit switch in said control sys-tem, said limit switch being responsive to the complete withdrawal of said neutron absorbing means to stop said motor.

4. The apparatus recited in claim 2 in which said control system includes a source of alternating current for said motor, a rectifier having an input connected to the source and an output connected to the electromagnetic clu-tch to provide a source of direct current, a release switch connected between the source and the rectifier for interrupting the energization of said clutch to uncouple *22 the motor from the neutron absorbing means and thereby permit the neutron absorbing means to be translated into the reactor by the gravity operated means.

5. In combination with a self-sustaining neutron chanreactor, a safety rod of neutron absorbing material, a gravity actuated means to Itranslate said safety rod along a path extending from the interior of the reactor to the exterior thereof, a drive means including a motor and chain drive for retrieving said rod from said reactor, an electromagnetic means for holding said rod in said retrieved position, means in said reactor responsive to a given maximum neutron density to control the electromagnetic means to release the gravity actuated means for inserting the safety rod into the reactor, and a manually operable device for deactuatng the electromagnetic means, thereby allowing said gravity actuated means to move said safety rod into said reactor.

6. The apparatus recited in claim 5 together with yieldable braking means for retarding said safety rod near the extreme limit of its path within said reactor.

7. The apparatus recited in claim 5 in which said electromagnetic means includes a catch insertable into a portion of said safety rod and held in such position when said electromagnetic means is energized, together with spring means for retrieving said catch from said rod portion when said electromagnetic means is deenergized.

8. In combination with a self-sustaining neutron chain reactor, a regulating rod of neutron absorbing material, means to translate said regulating rod along a path extending from the interior of the reactor to the ex-terior thereof including a first control means having a neutron responsive device disposed within said reactor, said control means controlling the position of Ithe regulating rod along its path to maintain the neutron density in said reactor at a substantially xed value, a safety rod of neutron absorbing material, means to translate said safety rod along a second path extending from the interior ofy the reactor to the exterior thereof including actuating means responsive to the neutron flux in the reactor to insert the safety rod into said reactor when a given maximum value of neutron density is reached, indicating means, generator means connected to said indicating means, said generator means being driven by said regulalting rod to control the indicating means, a plurality of circuits, each circuit having a microswitch positioned in the path of the safety rod, and a lamp, whereby the position of the safety rod is indicated by the lamps.

9. In combination with a neutronic reactor system including a reactor, means for maintaining the neutron density in said reactor at a relatively constant operating level comprising means for producing a response monotonically related to ythe deviation of the neutron density from an operating level, and means controlled by the magnitude of said response for changing the neutron reproduction ratio in the direction of unity including a regulating rod made of neutron absorbing material, means for translating the regulating rod along a path extending from the interior of the reactor to the exterior thereof, said last means including a pair of electric motors, an energizing circuit therefor including transformers having primary coils connected to the motors and secondary coils, means having tubes connected across each secondary coil for shunting said coil, and control means for controlling said last means to shunt said secondary coils and to effect drive of said motors, said control means including circuit means being responsive to change as well as rate of change of the response.

10. In combination with a neutronic reactor system including a reactor, means for maintaining the neutron density in the reactor at a relatively constant operating level comprising means for producing a response monotonically related to the deviation of the neutron density from said constant level, and means controlled by the magnitude of said response for changing the neutron reproduction ratio in the direction of unity, said last means'vcomprisingkae nent-ron absorbing control element,-

extending from tleeinterir" oflthe -j reactor lto theV exteriorthereof," 'am eleetrialcon-trol systemfor actuatingL said translatingrneans5to-'move thecontrol element alongfthe path-infeithfer of two directionsA at variable-'speedsfcomfprising apairI oi-v electric motors, ,ane energizingcircuit incldixgtransformershavingpri-mary coils connected to said=motors and secondary coils, means having vacuumv tubes-connected across the'seeondarycoils for shunting said-coils? and-*controlf means connected to the Vacuum tubcssfreffectin'g-control ofsaidV shuntingmeans, there-y Bix-elfe'ctiifi'g'the` drive-of said-motorsin certain directions,I a;sourcefolialternatingcurrent forA impressingv alternating cnrrentpotentials on s aidhshunting means, a phase-shifting means fory varying the phase" of said potentials, a source of variable direct" eur-rentpotential connected' to saidi'ph'ase shifting-means -forbiasingsaid' alternating current potentials;- said phase. shifting means -and'saidsource cooperating;toimpress'variable control potentials onthev'aeuumtub'es-'of tleshuntingvmeans'to effect variation on tlileil period ofcondctivity.

l'li Iii-combination wit-ha neutronic reactor system iny clndng` a' reactor',` means 'for maintaining theV neutron* densityat la'rela'tively constant operating level comprisingl means-for-prodncing a response monotonically rela-ted to thefdviation ofitheneutron density froma constant level, and'means controlled'bythe magnitudeof said response and'v bythe ratelotlchange of' saidv response for changingthe' neutron/reproduction'ratio in the direction ofunity, saldi last means includinga neutron absorbing control element; meansfo-r translating said control element alongy a*pathlextendi-ngy from the interior ofthe'reactor tothe exterior thereof; an velectrical control system for actuatingthetranslating meansT to move theV control element in saidreactor in either ofC two directions at variable speedsA comprising afpair--of `electric motors, an energizing'circuit therefor including qtran'sffme'shavingprimaryecoif and ization; chamber# and electronica' circuit'y connected-; tot the ionizationfchamb'er, said circuithaving` a tmeXre sponsiver cir'cuitinc-:hidinga condenser coupled between' the ionization chamber-1 andV the sli-unting means-,f

References-.Cited in tbe-dile of this patent UNITED STATES PATENTS! 353,305 Hennebergetal Nov.' 30;'. 1886 1,377,554` BlVC"JoIII` May"v 1921l 1,498,167 HillV Jne""17," 19.22%A 1,877,605 Shivers Sept: 13:1932-4v 1,9'15,()95- Jump Jlner20,f,1\933` 2,206,634 Fermin-a1; July 2; 19`40' 2,208,235 I Wli'te'nack' July 16,4 1940E- 2,37l,415 Toison Mr. 13j'- 1945Q 2,708,656 Fermifetiall lliayfl?,I 19-55 114,115.0 Australia May; 2'; 1940;?

114,151 fnxstral-ia May: 3, 1940- 23 3,011 Switzerland Oct; 2', 1944-4 OTH'ERI' REFERENCES Electric Lift Eqnigment for Modern Buildingsrwby Ronald" 'Griersong' pub; by Chapman lz'Hall,` Ltd, London (1923); Fignre'l ofthefr'ontispiece, pp; 53, 5'4, 55,' 143i' Smyth: Atomic Energyl for` Military Purposes?'ppjf 1771-180; August`11945.'

American] ournal of^`Physics,1 vol. 20, No. 9'(De'cembets 1952) pp, 536'l -andi'550l558. 

1. IN COMBINATION WITH A SELF-SUSTAINING NEUTRON CHAIN REACTOR, A SAFETY SYSTEM COMPRISING A NEUTRON ABSORBING MEANS, MEANS FOR TRANSLATING SAID NEUTRON ABSORBING MEANS ALONG A PATH EXTENDING FROM THE INTERIOR OF THE REACTOR TO THE EXTERIOR THEREOF COMPRISING MEANS EXERTING A FORCE UPON THE NEUTRON ABSORBING MEANS DIRECTED TOWARD THE INTERIOR OF THE REACTOR, MEANS FOR WITHDRAWING AND RESTRAINING SAID NEUTRON ABSORBING MEANS OUT OF SAID REACTOR INCLUDING A MOTOR, AN ELECTROMAGNETIC CLUTCH CONNECTED BETWEEN THE MOTOR AND THE FORCE EXERTING MEANS FOR COUPLING SAID MOTOR TO SAID NEUTRON ABSORBING MEANS, MEANS TO ENERGIZE SAID CLUTCH TO KEEP SAID NEUTRON ABSORBING MEANS OUTSIDE OF SAID REACTOR, AND MEANS TO DEACTUATE THE ELECTROMAGNETIC CLUTCH, THEREBY RESULTING IN THE INSERTION OF THE NEUTRON ABSORBING MEANS INTO SAID REACTOR. 